We are investigating the energetics and physical-chemical basis of the interactions that stabilize proteins. These studies include analyses of the stability of small, monomeric 13-sheet proteins in solution and also the role that sidechain-sidechain interactions play in modulating the rate of protein folding both in vivo and in vitro. In addition, our studies investigate the energetic balance and folding mechanism that determines whether a protein will be soluble or form amyloid fibrils in solution. We are also developing x-ray diffraction methods to gain high-resolution structural information on amyloid fibrils. The proteins we use for these studies are a series of variants of the B1 domain of IgG-binding protein G (beta1) and the Green Fluorescent Protein (GFP). Our studies with GFP have allowed us to design a novel system that has great utility for the detection of protein-protein interactions. We have dissected GFP into two halves that can only be reassembled if fused to two interacting proteins. We are using this system to investigate the affinity and specificity of protein-protein interactions. The specific example of tetratrcopeptide repeat (TPR) mediated protein-protein interactions is presented. The split GFP system also provides a means by which to identify binding partners for "orphan proteins" whose interaction partners are not yet known.